This invention relates to information signal communications systems, particularly telephone systems, employing a queuing feature.
In many information signal communication systems, particularly in telephone switching communication systems, several communication links are provided which are accessible to individual calling stations and which are grouped in different tariff or toll classes. For example, in the United States the Bell System provides communication links termed "trunk lines" of different classes which are leased at different rates, depending on the class. Specifically, Full Business Day (FBD), Wide Area Telephone System (WATS) trunk lines are available at one tariff schedule, measured time (MT) WATS trunk lines are available at a different tariff schedule, and Direct Distance Dialing (DDD) trunk lines are available at still another tariff schedule. The effect of such alternative tariff schedules is to reduce communication costs per call as the utilization of these trunk lines increases through the proper selection of trunk lines. The consequent motivation has been to load least expensive trunks and achieve cost savings as fully as possible subject to tolerable delays and inconveniences on the part of the user. In order to achieve higher utilization and consequent cost savings on less expensive communication links, e.g., the FBD WATS trunks, than a more expensive class of communication lines, e.g., the MT WATS trunk lines, such communications systems typically include a provision for automatically scanning FBD trunks first when a long distance call or non-local call is dialed by a calling station user associated to a particular communication switching system. In the case of telephones, for example, the telephone switchboard through which a caller's telephone is to be switched to the trunk lines performs this function.
An early type of scanning employed in telephone switching systems, termed "toll restriction without queuing" proceeds as follows. In response to the dialing of a non-local (long distance) telephone number by a caller, the switchboard circuitry scans the different groups of trunk lines in accordance with their tariff classes in accordance with a priority scheme, with the lowest tariff group of trunk lines being scanned first, the next lowest tariff class being scanned next, etc. The calling station is connected to the first available trunk encountered. Should the scanning circuitry determine the unavailability of any trunk line in any of the special lower tariff class groups, a busy signal is generated requiring the calling party to hang up and redial. As a specific example, in a telephone switching system having access to FBD and MT WATS trunk lines, and DDD trunk lines, the switchboard scans the FBD and MT WATS trunk lines only and generates a busy signal if no trunk in either of these two groups is available. This scanning suffers from the disadvantage that the accessibility of the WATS trunk lines decreases directly with increasing number of requests for access to such trunk lines, requiring frequent redialing during heavy periods of traffic and affording only random access to the lower tariff WATS trunk lines.
Due to the disadvantages of the "toll restriction without queuing" scanning technique, a more refined scanning system termed "toll restriction queuing," was developed. In this type of scanning system, scanning of the lower tariff class trunk lines initially proceeds in the manner noted above for "toll restriction without queuing." However, an unsuccessful scan, i.e., a scan during which no unoccupied FBD or MT WATS trunk line is available, results in the storage of a signal identifying the calling station desiring access to a lower tariff trunk line in a first-in, first-out (FiFo) storage device. Thereafter, the telephone switching equipment continuously scans the FBD and MT WATS trunk lines sequentially until a lower tariff trunk line becomes available, whereupon the calling station corresponding to the oldest identification signal stored in the FiFo is connected to the first available trunk line. Although the "toll restriction queuing" scanning technique frees the calling party from the necessity of frequent redialing in order to establish a lower tariff trunk line connection, in operation such a system introduces a delay in obtaining a trunk line connection (and thus in completing a long distance call) which is directly dependent upon the amount and number of such requests. Stated otherwise, the delay encountered in such a system is directly dependent upon the amount of traffic. Large traffic fluctuations in the system can still cause an inordinate wait and user inconvenience.
In order to limit the waiting period inherent in the "toll restriction queuing" technique, a further scanning improvement, termed "fixed time queuing," was developed. According to this scanning technique, the switching equipment initially scans the lower tariff trunk lines in a manner similar to that noted above, i.e., from highest priority to lowest priority class, in order to determine the availability of a lower tariff trunk line. If no available trunk is found during the scan, the calling station identification signal is stored in a fixed time queuing device, and the switching equipment continues to scan the grouped lower tariff trunk lines repetitively until a lower tariff trunk line becomes available. Should no lower trunk line become accessible within a predetermined maximum period of time, the calling station is connected automatically to a DDD trunk line and the call is then placed at regular rates. This scanning technique achieves cost savings by delaying traffic up to some maximum period of time before finally using more expensive DDD trunk lines in an attempt to seize the less expensive trunks. In a variation of the "fixed time queuing" technique, a second fixed time queue is inserted in the scanning chain downstream of the first fixed time queing device, typically between the last trunk line in the highest priority class and the first trunk line in the next highest priority class. In this arrangement, each unsuccessful scan of the highest priority trunk group causes the caller's identification number to be temporarily stored in the intermediate fixed time queue for a short period of time (e.g., 20 seconds), after which the caller'identification is retrieved and caused to scan the next highest priority trunk group. Should this scan be unsuccessful, the caller's identification is simply stored in the fixed time queue as noted above, with similar results. The intermediate fixed time queue gains economic savings over the single fixed time queue because of additional cost savings obtained by shifting traffic between the intermediate and the least expensive trunk lines in the group. However, daily traffic fluctuations often exceed the capacity of the least expensive trunk group, causing a considerable number of calls to have to overflow this intermediate fixed time queue in practical systems. During heavy periods of long distance traffic requests, the intermediate queue can get very large and many callers are unnecessarily required to wait until the predetermined time out period before gaining access to the next highest priority trunk group. Therefore, the maximum period of time specified for this intermediate queue must be chosen to be a small tolerable value. Having a small period of maximum wait in this intermediate queue restricts the cost savings which are achievable in practical communication systems.